Coating of automobile bodies is generally performed by applying an electrodeposition paint as the undercoat, thereafter applying an intermediate coat thereon, and further applying a top coat.
As coating methods, for example, 1-coat-1-bake system comprising applying a single kind of top coat and heat-curing the same; and 2-coat-1-bake system using two kinds of top coat consisting of base coat paint and clear coat paint, which comprises applying first the base coat paint, applying the clear coat paint thereon without curing the former, and heat-curing the two coating films simultaneously; may be named. Of these, in the 2-coat-1-bake system coating, it has been widely practiced to improve appearance of the coating film by using, as the base coat paint, one containing effect pigment such as aluminum flakes, mica or the like.
Effect pigment has sparkling brightness and changes its color tone with change in viewing direction, to exhibit unique decorative effect. This characteristic property of changing color tone according to viewing directions is referred to as flip-flop property, and effect pigment showing greater color tone change has a higher flip-flop property and contributes to superior appearance of the coating film containing the same.
In recent years, on the other hand, promotion of water-based paint is being advanced from the standpoints of environmental preservation and resource saving. Hence, also as to paint containing effect pigment, development of water-based base coat paint capable of forming coating film having excellent brightness is positively promoted.
Whereas, coating film formed of water-based base coat paint is subject to such problems as inferior flip-flop property or smoothness or occasional development of metallic mottling, compared to that formed of organic solvent-based base coat paint.
As a means to solve such problems, for example, JP Hei 2 (1990)-97564A discloses a method of improving flip-flop property of water-based base coat paint by having it contain fine particles of crosslinked polymer, but the problem of easy occurrence of metallic mottling in the formed coating film remains with this method.
Also JP Hei 5 (1993)-140485A discloses a method for inhibiting fat edge, sagging, metallic mottling and the like, by blending into water-based base coat paint montmorillonite and/or an organocomplex thereof in combination with silica powder, as thickener. This method, however, gives rise to a problem of tending to impair smoothness of coated surface presumably originating from deterioration in flow property (levelling property) of the paint.
Furthermore, JP 2001-104878A discloses a method for inhibiting film deficiency such as sagging, unevenness and the like occurring with change in coating conditions, in particular, in humidity, by using an aqueous dispersion obtained by copolymerization of specific long-chain monomers. In this method also the problem exists that degradation in smoothness of coated surface easily takes place, presumably due to deterioration in flow property of the paint.